Commonly known cryogen-cooled electrical machines of a conventional design configuration comprise a superconducting field winding placed in a hollow rotor and secured therein. The field winding is cooled by a coolant to the superconducting state, e.g., by liquid helium admitted inside the rotor and reducing the temperature of 4.3.degree. K.
To provide heat protection of the low temperature portion of the rotor, the latter is to be thermally insulated as required by heat conveyance from the ambient temperature zone to the cryogenic temperature zone. A vacuum space offers the best thermal insulating means for this case. To this end, an annular gap between the rotor and stator is evacuated. The loss of power in such an electrical machine will be lower, the higher vacuum is maintained in said gap. The best vacuum conditions are considered to be 10.sup.-5 torr.
One of the prior art cryogen-cooled electrical machines cf. U.S.S.R. Inventor's Certificate No. 547,926; Oct. 28, 1973) comprises a hollow stator and a rotor placed within the stator with an annular gap which is evacuated and kept under vacuum.
In the aforementioned electrical machine, a vacuum of 10.sup.-5 torr may be achieved only if the annular gap between the rotor and stator is connected via vacuum lines to a forevacuum and to a high-vacuum, e.g., turbomolecular, pumps. Such an evacuation system used to achieve and maintain the desired vacuum presents a number of self-contained units which, when combined with the electrical machine, make the overall design configuration rather complicated.
Besides, the efficiency of said system when used to achieve and maintain the desired vacuum in the electrical machine is considered low, because the lines communicating the gap with the pumps feature low throughput with respect to the rarefied gas. Moreover, accomplishment of vacuum-tight rotor shaft sealings has presented heretofore a serious design problem stemming from stringent requirements as to the purity of vacuum and high rotational speeds of the rotor shaft.